Permanent magnet structures that produce a working magnetic field are well known in the art. The term "working magnetic field" as used herein refers to a magnetic field that is used to do a useful task. A magnetic field used to guide or focus an electron beam is an example of such a working magnetic field.
Some permanent magnet structures are composed of pieces of permanent magnet material arranged to form a shell having an interior cavity. Each piece of permanent magnet material has a magnetization that adds to the overall magnetization of the shell. Depending on the magnetization of the shell, a permanent magnet structure can be designed to produce a working magnetic field having a given magnitude parallel to a given axis in the cavity of the shell.
Some permanent magnet structures are designed to provide a working magnetic field having a magnitude or strength that can be mechanically adjusted. Such structures are typically composed of two permanent magnet shells, each producing a working magnetic field in their respective cavities. The shells are arranged such that their internal cavities share a common space, thereby forming a common internal cavity. Such an arrangement enables the working magnetic fields to interact in the common internal cavity to produce, e.g. by vector addition, a composite working magnetic field having a composite magnitude. In addition, the shells are arranged such that they can rotate independent of each other around the common internal cavity. The "rotatability" enables one to change the vector relationship between the working magnetic fields produced by each shell. As a result, the magnitude of the composite working field in the common internal cavity can be adjusted or changed by rotating one shell with respect to the other.
One such adjustable permanent magnet structure is described in U.S. Pat. No. 4,862,128, entitled "Field Adjustable Transverse Flux Source," issued to the present inventor herein on Aug. 29, 1989, and incorporated herein by reference. As described therein, an adjustable working magnetic field can be obtained by assembling two cylindrical shells or "magic rings" such that one is embedded within the other so that they are axially aligned and the outer radius of the inner shell equals the inner radius of the outer shell. By rotating the shell of the outer magic ring around the inner magic ring, the magnitude of the working magnetic field in the cavity can be adjusted or changed without having to change the magnetization of either shell. Thus one has a structure that can be adjusted to produce a uniform magnetic field with any value in between, and including 2H.sub.1, where H.sub.1 is the field produced by each cylindrical shell acting alone.